The present invention relates to polypropylene.
Isotactic and syndiotactic polypropylene, and blends thereof, are known for use in a number of different applications. For example, polypropylene is used for the manufacture of spun fibres, blown films, extruded profiles and foams. In such applications in which the polypropylene is processed while molten, it is desirable for the polymer to have a high melt strength. For some applications, for example fibre spinning and film blowing, as well as having a high melt strength the polypropylene is required to have a high drawability. A high drawability not only enables the fibres or films to be produced at high speed without fracture, but also enables finer diameter fibres and thinner films to be manufactured.
There tends to be a compromise between high melt strength and drawability. Thus some known polypropylenes have high melt strength but low drawability. This makes them unsuitable for drawing fibres, particularly of small diameter.
A polymer melt having high melt strength at high shear rates refers to a melt that becomes stiffer and stronger when stretched, rather than one that thins out and breaks when stretched. This stiffening upon drawing is commonly called strain-hardening. Polypropylene processing operations where melt strength plays an important role include blow moulding, extrusion coating, thermoforming, fibre spinning and foam extrusion. In thermoforming, a poor melt strength results in a sagging phenomenon. In fibre spinning, a poor melt strength can result in undesired movements of the fibres due to transverse forces, for example by cooling air, which ultimately can lead to xe2x80x9cmarriedxe2x80x9d fibres and fibre breakage. On the other hand, a too-high melt strength will limit the achievement of low titre fibres. Accordingly, a correct balance between melt strength and drawability is desirable. For blown (biaxially oriented) or cast films also, a correct balance between melt strength and stretchability is very important. In foam extrusion, a poor melt strength results in cell rupture and non-uniform cell structure. For such an application, a poor drawability will limit the fineness of the walls.
Several solutions have been proposed in the prior art to increase the melt strength of polypropylene. For example, polymers with long chain branching tend to exhibit good melt strength. For isotactic polypropylene, this can be achieved by irradiation or by reactive extrusion processes, such as disclosed in U.S. Pat. Nos. 5,047,446, 5,047,485 and 5,541,236. The limitation of these processes is the significant reduction of drawability occurring at the same time as melt strength increases. In addition, the irradiation process is expensive. It has also been proposed to blend isotactic polypropylene with additives, such as high molecular weight acrylates, to increase the melt strength, as disclosed for example in EP-A-0739938. The same results can be achieved by blending with isotactic polypropylene polyethylene having high melt strength or fillers. These processes are limited by the strong modification by the additives of the intrinsic properties of the isotactic polypropylene.
It is also known from the literature that the melt strength of isotactic polypropylene is solely determined by its weight average molecular weight (Mw) (A. Gijsels Ind. Polym. Process., 9, 252 (1994)).
U.S. Pat. No. 5,549,867 relates to a melt spinning process for polyolefin resins in which a blended resin includes a relatively small portion of a low molecular weight high melt flow rate narrow molecular weight distribution polyolefin resin with a larger portion of a miscible high molecular weight, low melt flow rate and typically narrow molecular weight distribution polyolefin resin. It is disclosed that the enhanced molecular weight distribution polyolefin blended resin has a variety of property parameters, including a molecular weight distribution breadth Mz/Mn of between 7.2 and 10, a flow rate ratio of less than 15.5 and a power law index at 20 secondsxe2x88x921 of between 0.70 and 0.78 and either a Z-average molecular weight Mz of between 400,000 and 580,000, or a second order constant b2 determined from the regression analysis viscosity equation of between xe2x88x920.029 and xe2x88x920.047 or both, and unless both of the Mz and b2 parameters is within said ranges, a die swell B2 of between 1.6 and 2.0 and a spinnability factor ln (B2)/MFR of between about 0.08 and about 0.026.
U.S. Pat. No. 5,494,965 discloses a process for manufacturing bimodal olefin polymers and copolymers. However, the specification does not address the problems of drawing polypropylenes.
U.S. Pat. No. 5,578,682 discloses the bimodalisation of a polymer molecular weight distribution by using grafting and scission agents.
EP-A-0310734 discloses catalyst systems for producing polyolefin having a broad molecular weight distribution, in particular a multimodal molecular weight distribution. This specification does not address the problems of drawability of polypropylenes.
It is an aim of the present invention to provide polypropylene, which may be isotactic, syndiotactic or a blend of isotactic and syndiotactic fractions, which provides improved properties such as melt strength and drawability. It is also an aim of the present invention to provide such polypropylene which can be used in processing applications which require the polypropylene to be processed from the melt, for example at high shear rates, typically in fibre spinning. It is a further aim of the present invention to provide polypropylene which has an improved compromise between melt strength and drawability.
Accordingly the present invention provides the use of a multimodal isotactic polypropylene blend in melt processing wherein for enhancing a compromise between melt strength and drawability the blend has a dispersion index of at least 8 and a ratio Mz/Mn of at least 10.
The present invention also provides a method of enhancing a compromise between melt strength and drawability in melt processing of a polypropylene, the process including providing a multimodal polypropylene blend having a dispersion index of at least 8 and a ratio Mz/Mn of at least 10.
The present invention further provides a method of melt processing a polypropylene blend, the method comprising providing a multimodal polypropylene blend, selecting the blend to have a dispersion index of from 8 to 70 and a ratio Mz/Mn of at least 10 thereby enhancing a compromise between melt strength and drawability, and processing the blend in the melt by drawing the blend to form a solid product.
In this specification, the dispersion index (D) (also known as the polydispersity index) is the ratio between the weight average molecular weight (Mw) and the number average molecular weight (Mn). The ratio Mz/Mn is the molecular weight distribution breadth. Mz is the z-average molecular weight, defined as xcexa3NiMi3/xcexa3NiMi2 over all i.
The multimodal blend is preferably bimodal, but may alternatively be trimodal, tetramodal, etc. The blend of the fractions may be obtained by physical blending or chemical blending, for example chemical blending using two reactors in series or chemical blending using one reactor with specific dual-type catalysts. The polypropylene fractions may be composed of homopolymer or copolymer and may be made using differing catalysts, for example Ziegler-Natta catalysts or metallocene catalysts.
Preferably, the dispersion index is greater than 15. The dispersion index may be up to about 70.
The molecular weight distribution breadth is not especially limited, provided that it is 10 or above. Preferably, the molecular weight distribution breadth (Mz/Mn) is from 50-150.
The blend may comprise from 20 to 80 wt % of a first high molecular weight fraction and from 80 to 20 wt % of a second low molecular weight fraction.
Preferably, the blend comprises from 50 to 70 wt % of the first fraction and from 50 to 30 wt % of the second fraction. More preferably, the blend comprises from 55 to 65 wt % of the first fraction and from 45 to 35 wt % of the second fraction.
Preferably, the ratio of the melt flow indexes of the first and second fractions is at least 5. Typically, the first fraction has a melt flow index of less than 5 dg/min and the second fraction has a melt flow index of from 60 to 1000 dg/min.
Optionally, the blend has been formed by reactive extrusion of a mixture of the first and second fractions together with a mixture of a chain scission agent and a chain grafting agent. The chain scission agent may comprise 2,5-dimethyl-2,5-di(tert-butylperoxy) hexane. The chain grafting agent may be selected from allyl methacrylate and divinyl benzene.
The first and second fractions, and the blend, are preferably comprised of polypropylene homopolymer. Alternatively, the first and/or second fractions may se comprised of polypropylene copolymer.
The present invention also relates to the use of the polypropylene for forming fibres, foams, films, thermoformed articles and extruded products.